Air/fuel ratio control system

ABSTRACT

A method for controlling the air/fuel ratio of an internal combustion engine uses a manifold pressure sensor and an exhaust gas recirculation system having an orifice and a exhaust pressure sensor located upstream of the orifice. Both the manifold pressure sensor and the exhaust pressure sensor are sampled synchronously with a frequency proportional to the firing frequency of the engine and then filtered to eliminate unwanted frequencies. The sampled signals obtained then have the necessary bandwidth needed for closed loop control of the exhaust gas recirculation flow and engine air/fuel ratio control.

FIELD OF THE INVENTION

The present invention relates to an air/fuel ratio control system for aninternal combustion engine where air flow and exhaust gas recirculationflow are calculated from pressure sensors.

BACKGROUND OF THE INVENTION

Engine control systems often determine the amount of fuel to inject bymeasuring a manifold pressure, along with other engine operatingconditions. This method is often referred to by those skilled in the artas the speed density method. In this method, a mean value model ofengine operation is constructed, where an average manifold pressure at agiven speed results in a certain air flow into the cylinder. In thistype of system, measurement of the manifold pressure is critical forproper prediction of the air flow into the cylinder and thus for properair/fuel ratio control.

One approach for calculating a value of the manifold pressure to use inthe speed density approach is to sample the manifold pressure sensorwhen the piston is at top dead center, bottom dead center, and two otherpoints equally spaced from dead center positions. For example, the twoother equally spaced samples could be 60 degrees after dead center.Thus, four samples per revolution of the crankshaft are used that arenot necessarily equally spaced. Then, an average of the last two and thecurrent value of the manifold pressure is taken to obtain the averagedvalue of the manifold pressure used in the speed density method. Such asystem is disclosed in U.S. Pat. No. 5,497,329.

The inventors herein have recognized numerous disadvantages with theabove approach. For example, the sampling scheme described above willproduce a constant bias unless the two equally spaced samples occur inthe proper location. In particular, the resulting averaged manifoldpressure will be consistently offset from the true average. This resultsin an error in the prediction of air flow at a steady state operatingcondition. Another disadvantage, for example, is that the resultingaveraged manifold pressure will still contain oscillations that willcause cyclic errors in prediction of air flow at a steady stateoperating condition. These cyclic errors may cause reduced efficiency incontrolling regulated emissions.

Also, engine control systems relying on a manifold pressure sensor todetermine fresh charge entering the engine must be able to measure flowof exhaust gas recirculation to accurately control the exhaust air/fuelratio. Previous systems have used a differential pressure measurementacross an orifice to infer a flow of exhaust gas. Traditionally, theorifice is located upstream of the exhaust gas recirculation flowcontrol valve. Thus, the pressure measurements are shielded from theintake manifold pressure pulsations; however, the pressure measurementsare not shielded from the exhaust pressure pulsations. In thetraditional system, the high frequency pressure pulsations present inthe pressure measurements are reduced by using a conventional low passfilter. Such a system is disclosed in U.S. Pat. No. 5,613,479.

The inventors herein have recognized a significant opportunity to reducetotal system cost by relocating the orificedownstream of the exhaust gasrecirculation flow control valve but before the intake manifold. Thus,the manifold pressure sensor can be used to measure the pressuredownstream of the orifice and a single absolute pressure sensor can beused to measure the pressure upstream of the orifice. This creates theneeded differential pressure to measure exhaust gas recirculation flow.

The inventors herein have recognized numerous disadvantages with theabove approach. For example, the manifold pressure sensor is sensitiveto pressure fluctuations in the manifold and the upstream exhaustpressure sensor is sensitive to pressure fluctuations in the exhaustpressure. Since these fluctuations are out of phase with one another, asignificant error is created in the difference between the two. Anotherdisadvantage is the need for a conventional low pass filter to reducethese oscillations, where the conventional low pass filter is known tohinder transient performance.

SUMMARY OF THE INVENTION

An object of the invention claimed herein is to provide a method to moreaccurately calculate the fresh air entering a cylinder of an engine.

The above object is achieved, and disadvantages of prior approachesovercome, by a method for calculating air flow in an internal combustionengine. The method comprises sensing an engine speed of the engine,synchronously sampling a first pressure sensor with a frequencyproportional to a firing frequency of the engine, filtering saidsynchronously sampled first pressure with a filter to removeoscillations at frequencies proportional to said firing frequency, andcalculating a mass of gas entering a cylinder of the engine responsiveto said first filtered pressure and said engine speed.

By sampling the pressure waveform synchronously at a rate proportionalto the firing frequency of the engine and sampling with the properproportion, the pressure pulsations caused by the firing orders of theengine can be removed. This leaves the proper value of pressure, whichrepresents the mean value, for calculating air flow entering thecylinder. Thus, by properly selecting the location of sample pointsoccurring at a frequency proportional to the firing frequency of theengine, the inventors have obtained an unexpected benefit that not onlyeliminates all fluctuations in the filtered pressure, but also removesany constant bias.

In another aspect of the present invention both upstream and downstreampressures are sensed by sampling both the pressure waveformssynchronously at a rate proportional to the firing frequency of theengine and sampling with the proper proportion. Thus, the pressurepulsations caused by the firing orders of the engine can be removed.This leaves the proper constant value of differential pressure, whichrepresents the average value, for calculating exhaust gas recirculationflow entering the manifold.

An advantage of the above aspect of the invention is that the responseto transients in the mean pressure value is greatly improved.

Another advantage of the above aspect of the invention is improvedemission control.

Another advantage of the above aspect of the invention is that theaccuracy of the air charge calculation is increased.

Still another advantage of the above aspect of the invention isdecreased system cost.

Other objects, features and advantages of the present invention will bereadily appreciated by the reader of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

The object and advantages described herein will be more fully understoodby reading an example of an embodiment in which the invention is used toadvantage, referred to herein as the Description of the PreferredEmbodiment, with reference to the drawings wherein:

FIG. 1 is a block diagram of an engine in which the invention is used;

FIGS. 2-4 are high level flowcharts of various operations performed by aportion of the embodiment shown in FIG. 1; and

FIGS. 5-6 are examples of a fluctuating waveform on which the inventionis used to.

FIGS. 7A and 7B are plots showing frequency content of a pressure signaland an example of a notch filter's magnitude frequency characteristics.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT

Internal combustion engine 10 comprising a plurality of cylinders, onecylinder of which is shown in FIG. 1, is controlled by electronic enginecontroller 12. Engine 10 includes combustion chamber 30 and cylinderwalls 32 with piston 36 positioned therein and connected to crankshaft40. Combustion chamber 30 is shown communicating with intake manifold 44and exhaust manifold 48 via respective intake valve 52 and exhaust valve54. Intake manifold 44 is shown communicating with throttle body 58 viathrottle plate 62. Throttle position sensor 69 measures position ofthrottle plate 62. Exhaust manifold 48 is shown coupled to exhaust gasrecirculation valve 70 via exhaust gas recirculation tube 72. Exhaustgas recirculation valve 70 is also coupled to intake manifold 44 viaorifice tube 74. Orifice tube 74 has orifice 76 for restricting flowtherein. Intake manifold 44 is also shown having fuel injector 80coupled thereto for delivering liquid fuel in proportion to the pulsewidth of signal FPW from controller 12. Fuel is delivered to fuelinjector 80 by a conventional fuel system (not shown) including a fueltank, fuel pump, and fuel rail (not shown). Alternatively, the enginemay be configured such that the fuel is injected directly into thecylinder of the engine, which is known to those skilled in the art as adirect injection engine.

Conventional distributorless ignition system 88 provides ignition sparkto combustion chamber 30 via spark plug 92 in response to controller 12.Two-state exhaust gas oxygen sensor 96 is shown coupled to exhaustmanifold 48 upstream of catalytic converter 97. Two-state exhaust gasoxygen sensor 98 is shown coupled to exhaust manifold 48 downstream ofcatalytic converter 97. Sensor 96 provides signal EGO1 to controller 12which converts signal EGO1 into two-state signal EGO1S. A high voltagestate of signal EGO1S indicates exhaust gases are rich of a referenceair/fuel ratio and a low voltage state of converted signal EGO1indicates exhaust gases are lean of the reference air/fuel ratio. Sensor98 provides signal EGO2 to controller 12 which converts signal EGO2 intotwo-state signal EGO2S. A high voltage state of signal EGO2S indicatesexhaust gases are rich of a reference air/fuel ratio and a low voltagestate of converted signal EGO2S indicates exhaust gases are lean of thereference air/fuel ratio.

Controller 12 is shown in FIG. 1 as a conventional microcomputerincluding: microprocessor unit 102, input/output ports 104, read onlymemory 106, random access memory 108, and a conventional data bus.Controller 12 is shown receiving various signals from sensors coupled toengine 10, in addition to those signals previously discussed, including:engine coolant temperature (ECT) from temperature sensor 112 coupled tocooling sleeve 114; a measurement of manifold pressure (MAP) frommanifold pressure sensor 116 coupled to intake manifold 44; ameasurement of exhaust gas recirculation pressure (EGRP) from exhaustpressure sensor 117 coupled to orifice tube 74 upstream of orifice 76, aprofile ignition pickup signal (PIP) from Hall effect sensor 118 coupledto crankshaft 40, and an engine speed signal (RPM) from engine speedsensor 119. In a preferred aspect of the present invention, engine speedsensor 119 produces a predetermined number of equally spaced pulsesevery revolution of the crankshaft.

Referring now to FIG. 2, a flowchart of a routine performed bycontroller 12 to generate fuel trim signal FT is now described. Adetermination is first made whether closed-loop air/fuel control is tobe commenced (step 122) by monitoring engine operation conditions suchas temperature. When closed-loop control commences, signal EGO2S is readfrom sensor 98 (step 124) and subsequently processed in a proportionalplus integral controller as described below.

Referring first to step 126, signal EGO2S is multiplied by gain constantGI and the resulting product added to products previously accumulated(GI*EGO2S_(i-1)) in step 128. Stated another way, signal EGO2S isintegrated each sample period (i) in steps determined by gain constantGI. During step 132, signal EGO2S is also multiplied by proportionalgain GP. The integral value from step 128 is added to the proportionalvalue from step 132 during addition step 134 to generate fuel trimsignal FT.

The routine executed by controller 12 to generate the desired quantityof liquid fuel delivered to engine 10 and trimming this desired fuelquantity by a feedback variable related both to sensor 98 and fuel trimsignal FT is now described with reference to FIG. 3. During step 158, anopen-loop fuel quantity is first determined by dividing the differencebetween inducted mass air flow (AMPEM, created from the signal FMAP andRPM as described later herein with particular reference to FIG. 4),which includes both fresh charge and exhaust gas recirculation, andexhaust gas recirculation estimate (EM), which is described later hereinwith particular reference to FIG. 4, by desired air/fuel ratio AFd whichis typically the stoichiometric value for gasoline combustion. However,setting AFd to a rich value will result in operating the engine in arich state. Similarly, setting AFd to a lean value will result inoperating the engine in a lean state. Also, signal AMPEM is constructedfrom FMAP and RPM in the common speed density method known to thoseskilled in the art and can be easily empirically determined. Thisopen-loop fuel quantity is then adjusted, in this example divided, byfeedback variable FV.

After determination that closed-loop control is desired (step 160) bymonitoring engine operating conditions such as temperature (ECT), signalEGO1S is read during step 162. During step 166, fuel trim signal FT istransferred from the routine previously described with reference to FIG.2 and added to signal EGO1S to generate trim signal TS.

During steps 170, 172, 176, and 178, a proportional plus integralfeedback routine is executed with trimmed signal TS as the input. Trimsignal TS is first multiplied by integral gain value KI (step 170), andthe resulting product added to the previously accumulated products (step172). That is, trim signal TS is integrated in steps determined by gainconstant KI each sample period (i) during step 172. A product ofproportional gain KP times trimmed signal TS (step 176) is then added tothe integration of KI*TS during step 178 to generate feedback variableFV.

Calculating exhaust gas recirculation estimate (EM) is now describedwith particular reference to the diagram shown in FIG. 4. In particular,FIG. 4 shows how the upstream pressure (p1), which is signal EGRP inthis example, and downstream pressure (p2), signal MAP in this example,are processed to form the signal EM. First, in block 400, upstreampressure p1 is processed through a first filter known to those skilledin the art as an anti-aliasing filter with a cut-off frequency equal tof1. Similarly, in block 402, downstream pressure p2 is processed througha second anti-aliasing filter with a cut-off frequency equal to f2. Insome applications, it is unnecessary to use either the first or thesecond anti-aliasing filter because the geometry of the exhaust gasrecirculation creates a mechanical filter that removes the unwanted highfrequencies. Further frequencies f1 and f2 are set considerably higherthan the necessary control bandwidth.

Next, in block 404, the result of block 400 is synchronously sampledwith an engine rotation signal, such as, for example, RPM, such that thesampling is at a rate proportional to the firing frequency of theengine. For example the sampling rate could be twice the firingfrequency of the engine. The proportion is generally chosen such thatthe sampling is at a rate of twice the highest harmonic frequency thatcontains significant energy. Also, as would be obvious to one ofordinary skill in the art and suggested by this disclosure, any multipleof firing frequency greater than that determined above could be used.If, for example, the exhaust gas recirculation and engine geometry aresuch that higher order harmonics are present in the upstream pressuresignal p1, such as, for example, harmonics of twice or four times thefiring frequency, a sampling rate of four or eight times the firingfrequency may be necessary. Similarly, in block 406, the result of block402 is synchronously sampled with engine speed signal RPM, such that thesampling is at a rate proportional to the firing frequency of theengine. Additionally, it is not necessary that the sampling rate beequal in blocks 404 and 406. For example, block 404 could synchronouslysample at twice the firing frequency of the engine and block 406 couldsample at eight times the firing frequency of the engine.

Alternatively, as is obvious to one of ordinary skill in the art andsuggested by this disclosure, the pressure signal could be sampled at afrequency substantially proportional to the dominant frequency containedin the signal. This dominant frequency is usually equal to firingfrequency. Thus, sampling at a rate proportional this dominant frequencycould be accomplished using a circuit known to those skilled in the artas a phase-locked loop. However, because the phase locked loop sheme issometimes searching for the dominant frequency during transients, thisprocess may be suspended based on a change of position in throttle plate62. During the transition, an open loop estimate of how the change inthrottle plate 62 affects exhaust gas recirculation and manifoldpressure must be obtained. This can be done using a predetermined mapobtained through testing or analytical procedures and is known to thoseskilled in the art, where the transient behavior is estimated based onchange of position in throttle plate 62 and other operating conditions,such as for example engine speed.

Next, digital filters in blocks 408 and 410 process the results ofblocks 404 and 406. The digital filters, represented by G(z) or G'(z)used in blocks 408 and 410 are known to those skilled in the art asdigital notch filters. In this application, each notch filter removesthe firing frequency (and higher harmonics if necessary) of the engine.The equation below represents an example of a notch filter in thediscreet domain for sampling at a rate of twice the firing frequency.Use of notch filter G(z) is also described later herein with particularreference to FIG. 5.

    G(z)=(1+z.sup.-1)/2

If the sampling were done at a rate of eight times the firing frequency,then the following notch filter would be used as described by G'(z).Again, while this removes unwanted frequencies, transient performance isnot hindered. Use of a notch filter such as G'(z) is described laterherein with particular reference to FIGS. 6 and 7.

    G'(z)=(1+z.sup.-1 +z.sup.-2 +z.sup.-3 +z.sup.-4 +z.sup.-5 +z.sup.-6 +z.sup.-7)/8

The digital filter may be different between blocks 408 and 410 anddifferent than that shown above if necessary, such as if, for example,the geometry of the exhaust gas recirculation system was such that thecertain frequencies were excessively amplified due to resonances. Also,the filter may be different between blocks 408 and 410 if block 404synchronously sampled at twice the firing frequency of the engine andblock 406 sampled at eight times the firing frequency of the engine.

The pressure difference is then created by subtracting the output ofblock 410, which is filtered manifold pressure FMAP, from the outputfrom block 408. This pressure difference is then used in block 412 tocreate signal EM through a predetermined map or equation betweenpressure difference and exhaust gas recirculation flow, and, ifnecessary, engine operating conditions. For example, exhaust gastemperature may be used to adjust the calculation of exhaust gasrecirculation flow.

Also, in block 414, signals FMAP and RPM are used to calculate the massof gas flow entering the cylinder (AMPEM). The common speed densityequations known to those skilled in the art are used to convert thefiltered manifold absolute pressure with the engine speed to the totalmass of gas (exhaust gas and fresh air charge) entering the cylinder. Ifnecessary, these basic equations can be modified by engine operatingconditions, such as for example gas temperature, or any other conditionknown to those skilled in the art and suggested by this disclosure.

Thus, an estimate of the exhaust gas recirculation and fresh airentering the cylinder is obtained that is substantially free of unwantedfrequencies yet retains a bandwidth that is much greater than would beobtained with conventional filtering methods. Thus, the estimate canmore accurately track transient operation and yield more accurateair/fuel ratio control.

An example of synchronously sampling a waveform is now described withparticular reference to the plot shown in FIG. 5. A fluctuating pressuresignal, shown by the solid line and labeled A, is sampled with afrequency equal to twice the frequency of the actual signal. The sampledvalues are shown by points. The reconstructed waveform based on thesynchronously sampled values and the filter previously described hereinwith particular reference to the function G(z) is shown as the dottedline and labeled B. For comparison, a signal using a conventional lowpass filter, which is required for conventional sampling schemes, isshown by a dash dot line and labeled C. In this example, the exhaust gasrecirculation estimate formed using the synchronous sampling will yielda more accurate value that will allow for better overall air/fuel ratiocontrol.

Another example of synchronously sampling a waveform is now describedwith particular reference to the plot shown in FIG. 6. A fluctuatingpressure, shown by the solid line and labeled D, is sampled with afrequency equal to eight times the frequency of the lowest harmonicorder. This signal represents a typical exhaust pressure during steadystate operating conditions. The sampled values are shown by points. Thereconstructed waveform based on the synchronously sampled values and thefilter previously described herein with particular reference to thefunction G'(z) is shown as the dotted line and labeled E. This resultcould not be obtained unless the sampled values are all perfectly spacedwith the rotation of the engine, the synchronous sampling frequency wassuch that it was twice the highest significant harmonic frequency of thepressure signal, and the appropriate notch filter was used. In thisexample, the air flow entering the cylinder estimate formed using thesynchronous sampling will yield an accurate value that will allow foroptimal air/fuel ratio control.

Now referring to FIGS. 7A-7B and in particular to FIG. 7A, the plotshows the frequency content of the pressure waveform shown in FIG. 6.This pressure could represent, for example, the exhaust manifoldpressure for a steady state firing frequency of the engine ofapproximately 50 Hz. FIG. 7B shows a plot of the magnitude versusfrequency of the filter G'(z). Thus, the scheme previously describedherein with particular reference to FIG. 4, comprises (in the frequencydomain) multiplying the plots of FIGS. 7A and 7B. This shows that themean value, or DC component as known to those skilled in the art, ispreserved. The result is a signal substantially free of undesirablefrequencies for mean value model computations.

There are also other alternative embodiments of the present invention.For example, using a synchronous sampling scheme is not dependent on theorifice being located downstream of the exhaust gas recirculation flowcontrol valve. The scheme could be employed using a pressure sensorupstream and a pressure sensor downstream of the orifice, with theexhaust gas recirculation flow control valve still between thedownstream pressure sensor and the intake manifold, as in currentproduction vehicles. Furthermore, the method is not restricted to flowmeasurement with an orifice. Other flow measurement techniques known tothose skilled in the art could be used with the above described methodsuch as, for example, a venturi, a pitot tube, or a laminar flowelement.

While the best mode for carrying out the invention has been described indetail, those skilled in the art in which this invention relates willrecognize various alternative designs and embodiments, including thosementioned above, in practicing the invention that has been defined bythe following claims.

What is claimed is:
 1. A method for calculating air flow in an internalcombustion engine, the method comprising:sensing an engine speed of theengine; obtaining discrete pressure measurements from a continuouslymeasured first pressure at a frequency proportional to a firingfrequency of the engine; filtering said obtained discrete pressuremeasurements with a filter to remove oscillations at frequenciesproportional to said firing frequency; and calculating a mass of gasentering a cylinder of the engine responsive to said filtered discretepressure measurements and said engine speed.
 2. The method recited inclaim 1 wherein said first pressure sensor is a manifold pressuresensor.
 3. The method recited in claim 1 further comprising the step ofdetermining said frequency from an engine rotation signal.
 4. The methodrecited in claim 1 wherein said step of obtaining discrete pressuremeasurements further comprises the step of obtaining discrete pressuremeasurements from a continuously measured first pressure at said firstfrequency substantially proportional to a dominant frequency of saidfirst pressure measured by said first pressure sensor.
 5. The methodrecited in claim 4 wherein said step of calculating further comprisesthe steps of:suspending said calculation during a transition in a flowarea; and creating an open-loop estimate of said mass of gas responsiveto said transition and engine operating conditions.
 6. The methodrecited in claim 1 further comprising the steps of:obtaining seconddiscrete pressure measurements from a continuously measured secondpressure at a second frequency proportional to said firing frequency ofthe engine; calculating a gas flow estimate responsive to a differencebetween said discrete pressure measurements and said second discretepressure measurements.
 7. The method recited in claim 6 wherein saidsecond pressure sensor is an exhaust pressure sensor.
 8. The methodrecited in claim 6 further comprising the step of determining saidsecond frequency from an engine rotation signal.
 9. The method recitedin claim 6 wherein said gas flow is an exhaust gas recirculation flow.10. The method recited in claim 6 wherein said step of obtaining seconddiscrete pressure measurements further comprises the step of obtainingsecond discrete pressure measurements from a continuously measuredsecond pressure at said second frequency substantially proportional to adominant frequency of said second pressure measured by said secondpressure sensor.
 11. A method for calculating exhaust gas recirculationof an internal combustion engine having an exhaust gas recirculationtube coupled between an intake manifold and an exhaust manifold, anorifice in the tube, a first pressure sensor located upstream of theorifice, and a second pressure sensor located downstream of the orifice,said method comprising:obtaining first discrete pressure measurementsfrom a continuously measured first pressure of the first sensor at afirst frequency proportional to a firing frequency of the engine;obtaining second discrete pressure measurements from a continuouslymeasured second pressure of the second pressure sensor at a secondfrequency proportional to said firing frequency of the engine; andcalculating an exhaust gas recirculation flow responsive to a differencebetween said obtained first discrete pressure measurements and saidobtained second discrete pressure measurements.
 12. The method recitedin claim 11 further comprising the steps of:filtering said obtainedfirst discrete pressure measurements with a first filter to removeoscillations at frequencies proportional to said firing frequency;filtering said obtained second discrete pressure measurements with asecond filter to remove oscillations at frequencies proportional to saidfiring frequency; and calculating said exhaust gas recirculation flowresponsive to a difference between said obtained first filtered pressureand said obtained second filtered pressure.
 13. The method recited inclaim 11 further comprising the step of determining said first freqeucnyand said second frequency from an engine rotation signal.
 14. The methodrecited in claim 11 wherein said step of obtaining first discretepressure measurements further comprises the step of obtaining firstdiscrete pressure measurements from a continuously measured firstpressure of the first sensor at said first frequency substantiallyproportional to a first dominant frequency of said first pressuremeasured by the first pressure sensor.
 15. The method recited in claim11 wherein said step of obtaining second discrete pressure measurementsfurther comprises the step of obtaining second discrete pressuremeasurements from a continuously measured second pressure of the secondsensor at said second frequency substantially equal to a second dominantfrequency of said second pressure measured by the second pressuresensor.